This invention relates to electrical power cables which have concentric neutral wires (CN wires) applied helically over the cable core as a metallic ground shield which is then covered by a protective polymeric jacket. More particularly, the invention relates to an improved protection against migration of water in such power cables by providing suitable continuous, elongated water swellable elements, such as yarns, filaments, strands or strips in contact with the CN wires and so disposed in relation to said CN wires as to block the passage of water within the cable in the longitudinal direction.
A power cable is a long, cylindrically symmetric structure with a dielectric which operates at relatively high electrical stress. It normally consists of a metallic conductor covered with a semi-conductive shield (i.e. conductor stress control layer), over which a solid dielectric (i.e. insulation), such as a crosslinked polyethylene (XLPE) or ethylene propylene rubber (EPR), is extruded. This solid dielectric is covered with a semi-conductive shield (i.e. insulation stress control layer), thus forming a cable core on which a metallic ground shield is applied. The preferred metallic shield, particularly for medium voltage underground distribution cables, consists of CN wires applied helically over the cable core and over which an overall polymeric jacket is extruded to provide protection against radial moisture ingress into the insulation. The grounded metallic shield serves the following functions: (a) to provide a neutral current return path and to ensure that the outside surface of the cable insulation is at ground potential; and (b) to provide a preferred path to ground for any fault currents and to ensure tripping of protective devices.
It is known that moisture ingress into the insulation can result in the formation of "water trees" which shorten cable life significantly. Water trees are diffused structures or micro-channels with a bush like or fan-like appearance. They grow from defects such as voids, contaminants and semi-conductive shield protrusions in the presence of water and an electric field. The overall protective polymeric jacket, provided over the metallic shield, has a positive effect in minimizing tree growth. However, buried, underground distribution cables sometimes experience mechanical damage to the jacket during installation or subsequent accidental dig-ins, allowing ground water to migrate under the jacket. This almost unlimited supply of water can result in the accelerated growth of water trees in the affected section of the cable. In addition, the length of cable exposed to this accelerated tree growth is increased due to water migration along the longitudinal axis of the cable. Obviously, the probability of cable failure will increase as the length of the affected section increases. One approach for limiting the affected area is to use an encapsulating jacket over the concentric neutral (CN) wires to minimize longitudinal water migration over the entire length of the installed cable. Unlike the conventional "sleeve" jacket, the encapsulating jacket is designed to fill the spaces between the concentric neutral wires. While the encapsulating jacket is an improvement over the "sleeve" jacket in terms of resistance to longitudinal water migration, it is not entirely effective in that some water leakage occurs along the slight grooves or indentations made by the concentric neutral wires and/or at the interface between the cable core and the jacket. The water leakage can be observed when the cable is tested in accordance with the water penetration test procedure specified in industry specifications such as International Electrotechnical Commission (IEC) 840 (Amendment 1).
It is already known to use a water swellable material in an electrical power cable to provide a water barrier under the jacket of such cable. For example, U.S. Pat. No. 5,010,209 issued Apr. 23, 1991, discloses use of water swellable particles, namely powder, or of a filling compound with such particles or of a water swellable tape or a combination of these to provide such barrier. However, in the construction using CN wires, referred to as wire serving, as shown in FIGS. 6 to 8 of said U.S. Pat. No. 5,010,209, a layer of water swellable particles is always provided. The use of water swellable powder presents a number of disadvantages. When working with such powder, dust particles are spread in the air and they may cause a flash fire in the presence of a flame. Such dust may also cause respiratory problems and/or eye irritation. Moreover, surfaces subject to spills or dusting can become slippery when wet, resulting in unsafe work areas.
The use of a layer of water swellable tape over the length of the cable increases the overall diameter and weight of the cable which, in many instances, is undesirable. Also, the cost associated with the application of water swellable tape and powder is significant and will translate into a higher cost of the cable.
U.S. Pat. No. 5,146,046 issued Sep. 8, 1992 discloses the use of two water swellable strand-like members, such as yarns, wrapped in opposite helical directions between the relatively supple core wrap layer and the smooth, relatively rigid jacket of a communication cable. The major difference between the communication cable of U.S. Pat. No. 5,146,046 and the electrical power cable of the present invention is that the latter requires the use of CN wires and of a protective plastic jacket as part of the insulation shield system. The use of strand like members such as shown in U.S. Pat. No. 5,146,046, in a communication cable without the CN wires, does not provide any indication of water blocking capability of such strands in a power cable with a ground shield consisting of CN wires.